Photolytically and environmentally stable multilayer structure for high efficiency electromagnetic energy conversion and sustained secondary emission

ABSTRACT

A multilayer structure for authentication that includes an energy conversion layer, at least one stability enhancement layer and at least one optical variable element is disclosed. Also disclosed are methods of creating and using the inventive multilayer structure for authentication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/249,350 filed Sep. 30, 2011, titled, “Photolytically andEnvironmentally Stable Multilayer Structure for High EfficiencyElectromagnetic Energy Conversion and Sustained Secondary Emission,”which in turn claims priority to U.S. Provisional Patent ApplicationSer. No. 61/388,040, filed Sep. 30, 2010 of the same title, each ofwhich are incorporated by reference herein in their entirety for allpurposes.

BACKGROUND

These teachings relate generally to electromagnetic radiation convertingstructures, and, more particularly, to the preparation and use of highquantum efficiency primary electromagnetic energy converting structuresthat are stable, and hence capable of providing sustainable secondaryemissions, that is, for long durations at desired wavelengths.

Desirable electromagnetic emissions at specific wavelengths can beachieved by either designing the primary emitter to emit at the desiredwavelength, such as, for example, different compositionelectroluminescent devices, or the primary emitter can emit at a primarywavelength that is shorter, often at a higher efficiency, which emissioncan then be down-converted to the desired longer wavelength by an energyconverting element or material. For example, Photoluminescent Materials(e.g. High Persistence Phosphors, such as those disclosed in U.S. Pat.No. 6,117,362 (Blue), or in U.S. Pat. No. 6,267,911 (Green)),Electroluminescent Materials (e.g. UV or blue Light Emitting Diodes(LEDs), or lasers, etc.), Chemiluminescent Devices, etc., can emit at ashorter primary emission wavelength, which emission can bedown-converted to desired secondary emissions having longer wavelengths.

Applications of fluorescent dye-based energy converting compositions arecited in U.S. patent application Ser. No. 11/793,376, filed on Dec. 20,2005, for down-converting primary emissions of high-persistencephosphors to other longer-wavelength visible emissions. Therein thefluorescent dye-based energy conversion structures are rendered as filmswhich embody both the primary emission source (a High PersistencePhosphor), and the energy conversion element in a multilayer filmelement, such film structures having the flexibility to be applicable toany object.

Use of short-persistence phosphorescent materials are cited in U.S. Pat.No. 7,151,283 for down-converting UV, blue or green LED, primaryemissions into secondary emissions at desirable wavelengths, such as,for example, white light. Therein the phosphorescent conversion elementis located within the solid state device.

Uses of organic fluorescent materials for down-converting shorterwavelength radiation emitted by UV, blue, or green LEDs to desiredlonger wavelength radiation are cited in U.S. Pat. No. 6,600,175.Therein also the fluorescent-dye conversion element is located withinthe solid state device.

Uses of fluorescent materials for down-converting primary emission fromUV, or blue, or green LEDs, are also cited in “High EfficiencyPhosphor-Converted Light Emitting Diodes for Solid State Lighting” bySteven Allen, Ph.D. Thesis, Univ. of Cincinnati. For the devicesproposed in the latter citation the fluorescent organic fluorescentdye-based conversion element is located outside the LED solid statedevice.

High-Persistence inorganic phosphorescent material based products can beseen in airplanes, multistoried buildings, institutions, etc. Suchproducts do not use any energy conversion elements, and emit at theirprimary emission wavelengths that are located in the blue (˜490 nm) orgreen (˜515 nm) areas of the visible spectrum. Remarkably, in spite ofthe availability of a broad class of high quantum efficiency organicfluorescent materials that can be used as energy conversion elements tocreate other colors, such products are not found in the marketplace.Apart from the desirability of having photo-luminescent productsemitting in other colors (wavelengths) for aesthetical reasons, there isa need for other emission colors for serving informational purposes. Anexample is a position and hold bar (stripe) at airports, where both thedaytime and nighttime (emissive) colors of such a marking need to be inthe yellow region of the visible spectrum.

In the world of semiconductor based electroluminescent devices, such asLEDs, there has been a big push to create white light emitting LEDs.This is generally accomplished by utilizing a UV or blue primaryemission LED and down-converting their primary emission into white lightwith an energy conversion element that is an inorganic short-persistencephosphorescent material, such as cerium doped yttrium aluminate. Theinorganic phosphorescent energy conversion element is located inside thesolid state device. Inorganic phosphorescent materials, such as ceriumdoped yttrium aluminate, are generally photolytically stable and henceare capable of providing sustainable emissions over a long lifetime.Although other phosphorescent materials are known, which either alone orin combination are capable of down-converting to other colors, their usehas not gained wide acceptance in the marketplace. This is because highefficiency materials that can generate the wide range of colors are notavailable. Similarly, the spectrum of whites from cool white to warmwhite cannot be created.

Even though the down-converting properties of organic fluorescentmaterials have been known for some time, the use of such materials asenergy conversion elements has been precluded by their inability toprovide sustainable secondary emissions over long periods of time, foreither outdoor applications using phosphorescent materials as theprimary radiation, or for applications using LEDs or lasers as primaryradiation. This is because so far the conversion elements have not beenphotolytically or thermally stable over the long time horizons requiredfor commercial applications.

Multilayer film structures, such as those cited in U.S. patentapplication Ser. No. 11/793,376, filed on Dec. 20, 2005, can be deployedin a manner wherein the primary emission source, such as the highpersistence phosphorescent material, can be located within the filmstructure, either as a separate layer or within the same layer whereinthe organic fluorescence conversion element is located. However, testingreveals that the structures cited do not have long term stability,either for multiple year outdoor usage or for sustainable emissions forlifetimes that are generally desired for solid state devices.

Similarly, fluorescent organic energy conversion elements, such as thosecited in U.S. Pat. No. 6,600,175, wherein the conversion element islocated within a solid state device, are not capable of sustainedemission over the lifetime that is generally required for solid statedevices due to degradation of the conversion elements. It should benoted that inside the LED both the light intensity and temperatures arehigh (temperatures typically exceed 100° C. and can reach 150° C.),thereby accelerating both photolytic and thermal degradation of theorganic fluorescent energy conversion elements.

Locating the conversion element remotely from the solid state device,such as an LED, will generally result in its experiencing a loweroperating temperature. This may benefit emission sustainability byretarding the rate of degradation of the material. Nevertheless, evenwith remote location, the organic fluorescent energy conversion elementis not capable of sustained emission over lifetimes typically requiredof solid state devices due to degradation. Remote location of the energyconversion element would also permit greater form factor flexibility byfacilitating creation of different form factor lighting devices. Thus,depending upon the application, whether the film based energy conversionelements are located within or remotely from the solid state device, itwould be beneficial to have a film based energy conversion structuresthat allow for sustained emissions.

Fluorescent or phosphorescent conversion elements, such as those citedabove, are Lambertian emitters and therefore emit light in alldirections. Since the emissions are generally viewed from one direction,it can be appreciated that it would be beneficial to redirect theemitted light towards the viewer. In cases where the film-based energyconversion structures are exposed to the environment, it would also bebeneficial to provide clear protective layers for protecting the energyconversion materials from the environment.

Today's white light LEDs utilize inorganic phosphorescent materials thatare specific for energy conversion of primary radiation (typically blueor UV) to white light. The use of fluorescent energy conversionelements, such as organic fluorescent dyes, has the potential toincrease the conversion efficiency compared to that achieved withphosphorescent materials. Even with the incentive for attaining higherenergy conversion efficiency, the use of fluorescent dye-basedconversion elements has been precluded for generating sustained emissionbecause of instability of these energy conversion elements, that is,sustained emissions cannot be achieved.

There is, therefore, a need for utilizing higher quantum efficiencyenergy conversion elements, such as organic fluorescent dyes, that arerendered in structures which are stable and capable of sustainedemissions over the lifetimes desired for various applications. It isalso desirable, therefore, to provide for energy conversion elementsrendered as multilayer structures, wherein the structure not onlyembodies elements for achieving the high efficiencies in converting theprimary emission, but also embodies elements that can substantiallyincrease both photolytic and thermal stability of energy conversionelements, so as to enable sustained emissions over long periods of time.Furthermore, it is also desirable to incorporate within these structureslayers that provide the means to direct the Lambertian emissions forwardfrom the conversion elements into the hemisphere from which theemissions will be viewed, as well as protect the conversion elements,both physically (abrasion, etc.) and chemically (solvents, moisture,etc.), from the environment.

Energy conversion elements that provide sustained emission over longperiods of time can have substantial utility in a number of differentareas, such as in conjunction with primary emissions emanating fromshort or long persistence phosphors, electroluminescent devices, such asfluorescent tubes, LEDs, lasers, chemo-luminescent devices, LCDs etc.Such elements can be used not only for providing illumination inconsumer and industrial applications, but also for displays.Furthermore, these energy conversion elements can also be used forauthentication of items such as, credit cards, identification cards,transit passes or any item requiring authentication. There is a need forutilizing these energy conversion elements that are rendered inmultilayer structures in order to provide unique charging/dischargingand emission characteristics that can be tailored at the site ofmanufacture and that are difficult to counterfeit.

SUMMARY

The present teachings provide for a multilayer structure for sustainedenergy conversion of a primary electromagnetic radiation. The multilayerstructure comprises an energy conversion layer which converts the energyof the primary electromagnetic radiation. The energy conversion layercomprises a polymer and a first photoluminescent material. The firstphotoluminescent material is characterized by a first Stokes shift and afirst radiation absorption spectrum. The first radiation absorptionspectrum at least partially overlaps with the spectrum of the primaryelectromagnetic radiation. Typically in the energy conversion layer,successive energy conversion occurs through successive energy conversionmaterials that are characterized by overlapping emission and absorptionspectra. It is not required in all instances to cascade energy through aseries of emission and reabsorption steps, and one may achieve theresultant energy conversion by way of Förster transfer. The multilayerstructure further comprises at least one stability enhancement layer.The stability enhancement layer increases the photolytic and thermalstability of the energy conversion layer. The polymer in the energyconversion layer of the multilayer structure of the present teachings isselected to be photolytically and thermally stable. In certainconstructions, the function provided by the stability enhancement layercan be incorporated within the energy conversion layer itself. Themultilayer structure of the present teachings may also comprise aselective reflection layer which redirects radiation emitted in theenergy conversion layer, for example in the direction where in may beperceived. The reflection layer, where applicable, is also capable oftransmitting at least a portion of the primary electromagneticradiation. The multilayer structure of the present teachings mayadditionally comprise a diffusion layer that increases the opticalscattering of the pumping radiation and/or the emitted radiation in theenergy conversion layer. The multilayer structure of the presentteachings may also comprise a protective layer which provides mechanicaland chemical durability for the multilayer structure. The protectivelayer may also include phase change materials to lower the operatingtemperature of the multilayer structure, thereby enhancing thedurability of the multilayer structure. The primary electromagneticradiation may emanate from within the multilayer structure itself. Forexample, the energy conversion layer may further comprise a highpersistence photoluminescent material and the primary electromagneticradiation may emanate from this high persistence photoluminescentmaterial. In another aspect, the multilayer structure may furthercomprise at least one electroluminescent layer, such as an organic lightemitting diode, and the primary electromagnetic radiation may emanatefrom this electroluminescent layer. In another aspect, the multilayerstructure may further comprise at least one electroluminescent phosphorfrom which the primary electromagnetic radiation may emanate. In anotheraspect, the multilayer structure may further comprise at least onechemiluminescent layer, from which the primary electromagnetic radiationmay emanate. Alternatively, either ambient light can provide the primaryelectromagnetic radiation or the primary electromagnetic radiation mayemanate from an electroluminescent source, or from a solid state device,or from a chemiluminescent source. The first photoluminescent materialof the energy conversion later may comprise an organic fluorescent dye.The multilayer structure of the present teachings may further comprise asecond photoluminescent material. The second photoluminescent materialis characterized by a second Stokes shift and a second radiationabsorption spectrum. The second radiation absorption spectrum at leastpartially overlaps with the spectrum of the radiation emission of thefirst photoluminescent material.

The present teachings also provide for a method for sustained energyconversion of a primary electromagnetic radiation. The method of thepresent teachings comprises providing a multilayer structure forsustained energy conversion of a primary electromagnetic radiation. Themultilayer structure comprises an energy conversion layer which convertsthe energy of the primary electromagnetic radiation. The energyconversion layer comprises a polymer and a first photoluminescentmaterial which is characterized by a first Stokes shift and a firstradiation absorption spectrum. The first radiation absorption spectrumat least partially overlaps with the spectrum of the primaryelectromagnetic radiation. Typically in the energy conversion layer,successive energy conversion occurs through successive energy conversionmaterials that are characterized by overlapping emission and absorptionspectra. It is not required in all instances to cascade energy through aseries of emission and reabsorption steps, and one may achieve theresultant energy conversion by way of Förster transfer. The multilayerstructure further comprises at least one stability enhancement layerwhich increases the photolytic and thermal stability of the energyconversion layer. The multilayer structure may further comprise areflection layer which redirects radiation emitted in the energyconversion layer, for example in the direction where in may beperceived. The multilayer structure may also further comprise adiffusion layer which increases the optical scattering of the pumpingradiation and/or the emitted radiation in the energy conversion layer.The multilayer may also comprise a protective layer to enhancedurability of at least the energy conversion layer. The method of thepresent teachings further comprises exposing the energy conversion layerto the primary radiation source. The method further comprises convertingenergy from the primary radiation source to a longer output wavelength.The source of the primary electromagnetic radiation of the method may beprovided within the multilayer structure itself. Alternatively, thesource could be located remotely. For example, ambient light could alsoserve as the primary electromagnetic radiation. In other examples, theprimary electromagnetic radiation may be provided from anelectroluminescent source, or form a solid state device, or from achemiluminescent source. To provide the source of the primaryelectromagnetic radiation, the multilayer structure may be placed insideor in close proximity to a solid state device.

The present teachings further provide for a method of forming amultilayer structure for sustained energy conversion of a primaryelectromagnetic radiation. The method comprises forming an energyconversion layer comprising a first photoluminescent material which ischaracterized by a first Stokes shift and a first radiation absorptionspectrum. The first radiation absorption spectrum overlaps with thespectrum of the primary electromagnetic radiation. Typically in theenergy conversion layer, successive energy conversion occurs throughsuccessive energy conversion materials that are characterized byoverlapping emission and absorption spectra. It is not required in allinstances to cascade energy through a series of emission andreabsorption steps, and one may achieve the resultant energy conversionby way of Förster transfer. The method further comprises overlaying atleast one stability enhancing layer and, optionally, a protective layerover the energy conversion layer. The stability enhancing layer, orlayers, increases the photolytic and thermal stability of the energyconversion layer, and the protective layer protects the multilayer filmstructure physically and chemically. The method may further compriseapplying a reflection layer to the energy conversion layer to redirectradiation emitted in energy conversion layer. The method mayadditionally comprise applying a diffusion layer to the energyconversion layer to increase the optical scattering of the pumpingradiation and/or the emitted radiation in the energy conversion layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teachings are illustratively shown and described inreference to the accompanying drawings, in which

FIG. 1 is a schematic that describes the cascade of emission from onephotoluminescent material to another;

FIG. 2A is a schematic drawing of an energy converting structureaccording to one aspect of these teachings;

FIG. 2B is a schematic drawing of an energy converting structureaccording to a second aspect to these teachings;

FIG. 3 is a schematic illustration of multi-layer particleconfigurations: (a) Energy conversion layer (ECL) without any outerlayers; (b) ECL with Stability Enhancement Layer; (c) ECL with StabilityEnhancement Layer and Protective Layer; (d) ECL with Protective Layer;(e) Core Layer with ECL layer; (f) Core Layer with ECL and StabilityEnhancement Layer; (g) Core Layer with ECL, Stability Enhancement Layer,and Protective Layer; (h) Core Layer with ECL and Protective Layer; (i)Core Layer with Diffusion Layer and ECL; (j) Core Layer with DiffusionLayer, ECL and Stability Enhancement Layer; (k) Core Layer withDiffusion Layer, ECL, Stability Enhancement Layer and Protective Layer;(l) Core Layer with Diffusion Layer, ECL and Protective Layer; (m) CoreLayer with Reflection Layer and ECL; (n) Core Layer with ReflectionLayer, ECL, and Stability Enhancement Layer; (o) Core Layer withReflection Layer, ECL, Stability Enhancement Layer, and ProtectiveLayer; (p) Core Layer with Reflection Layer, ECL, and Protective Layer;(q) Core Layer with Reflection Layer, Diffusion Layer and ECL; (r) CoreLayer with Reflection Layer, Diffusion Layer, ECL and StabilityEnhancement Layer; (s) Core Layer with Reflection Layer, DiffusionLayer, ECL, Stability Enhancement Layer and Protective Layer; (t) CoreLayer with Reflection Layer, Diffusion Layer, ECL and Protective Layer;

FIG. 4 is a diode-illuminated waveguide structure utilizing a multilayerenergy converting structure according to these teachings;

FIG. 5 is a diode array utilizing multilayer energy convertingstructures according to these teachings;

FIG. 6 is a multilayer energy converting sleeve according to theseteachings;

FIG. 7 is a graph illustrating the effects of using a protective layerwith and without a stability enhancement layer on the degradation ofphotoluminescent materials;

FIG. 8 is a graph comparing the degradation of photoluminescentmaterials with and without the use of a stability enhancement layer;

FIG. 9 is a graph illustrating the effects of using a protective layerand a stability enhancement layer on the degradation of photoluminescentmaterials;

FIG. 10 shows the spectrum of a cool white emission generated from afilm prepared according to these teachings; and

FIG. 11 is a schematic that describes the energy transfer from onephotoluminescent material to another by way of Förster transfer.

DETAILED DESCRIPTION

The present teachings are directed to the creation of photoluminescent,high quantum efficiency energy conversion structures with one or morelayers rendered as cast, extruded, coated films, planar or otherwise, orsingle or multilayer particles comprising energy conversion elementswith preference for such elements being organic fluorescent dye basedstructures that can convert primary radiation, or primary emissions ofshort wavelengths, such as those emitted by phosphorescent materials ofhigh and low persistence, electroluminescent devices, such as LEDs,lasers, electro-luminescent devices based on short and long persistencephosphors, chemiluminescent devices, and the like, to longerwavelengths.

For a better understanding of the disclosure the following terms areherein defined:

The term “luminescence” is defined as the emission of electromagneticradiation from any substance. Luminescence occurs from electronicallyexcited states.

The electronic excited states of most organic molecules can be dividedinto singlet states and triplet states.

As used herein, the term “singlet state” refers to an electronic statewherein all electrons in the molecule are spin-paired.

As used herein, the term “triplet state” refers to an electronic statewherein one set of electron spins is unpaired.

The excited state is usually the first excited state. A molecule in ahigh vibrational level of the excited state will quickly fall to thelowest vibrational level of this state by losing energy to othermolecules through collision. The molecule will also partition the excessenergy to other possible modes of vibration and rotation.

“Luminescent materials” are those which exhibit luminescence, that is,emit electromagnetic radiation. Characterizing luminescent materialsrequires consideration of: (1) the excitation source, (2) the nature ofthe emission, and (3) whether or not additional stimulation is requiredto cause emission.

With regard to the excitation source, luminescent materials excited byelectromagnetic radiation are referred to herein as “photoluminescent.”Luminescent materials excited by electrical energy are referred toherein as “electroluminescent.” Luminescent materials excited by achemical reaction are referred to herein as “chemiluminescent.”

With regard to the nature of the emission, this may be eitherfluorescence or phosphorescence. A “fluorescent” material storeselectromagnetic radiation and releases it rapidly, generally in about10⁻⁸ seconds or less, in a process that does not invoke a change in theelectronic spin state of the molecule. Fluorescence from organicmolecules typically occurs from excited singlet states. Contrarily, a“phosphorescent” material stores electromagnetic radiation and releasesit gradually, in about 10⁻⁶ seconds or greater, in a process thatrequires a change in the electronic spin state of the molecule.

“Primary Radiation” refers to electromagnetic radiation of the shortestaverage wavelength from a luminescent element. Primary radiationgenerally needs to be converted into a radiation of a longer wavelength.

“Pumping Radiation” is a term used herein synonymously with “PrimaryRadiation”.

“Primary Excitation” refers to electromagnetic radiation that excites amolecule from a lower energy state to a higher energy state. “PrimaryExcitation” may in some instances, be synonymous with “PrimaryRadiation”.

“Energy conversion element” refers to a photoluminescent materialcapable of converting a primary radiation into a secondary radiation ofa longer wavelength.

“Liquid carrier medium” is a liquid that acts as a carrier for materialsdistributed in a solid state and/or dissolved therein.

As used herein, a “formulation” is a liquid carrier medium, as definedabove, comprising at least one polymer material either dissolved inand/or distributed in a solid state within said liquid carrier medium.

A “dispersion” is a formulation, as defined above, which additionallycomprises a material that is a solid distributed in the liquid carriermedium.

A “solution” is a homogeneous mixture of at least two materials. Asolution may be a formulation wherein an ingredient, such as a dye or apolymer, is dissolved in the liquid carrier medium, or a homogeneousmixture of a dye in a polymer.

A “solid state solution” of dye and polymer is a homogeneous mixture ofthe two in the dry state. One possible way of achieving that is themixture resulting from applying a formulation comprising the said dyeand polymer in solution in a liquid carrier and drying (removingsolvent) the said dye and polymer solution. Such a homogeneous mixturemay also result from subjecting a mixture of said dye and polymer toelevated temperatures. Note that for a homogeneous mixture to form in adry state, the polymer and dye have to be compatible, otherwise ahomogeneous mixture will not result.

A “photoluminescent formulation” is a formulation, as defined above,which comprises a photoluminescent material as defined above.

A “phosphorescent formulation” is a formulation, as defined above, whichcomprises a phosphorescent material as defined above.

A “fluorescent formulation” is a formulation, as defined above, whichcomprises a fluorescent material as defined above.

A “protective formulation” is a formulation, as defined above, whereinsome or all of the polymeric resin and/or additional materials areselected for providing protection against photolytic degradation and/orfor providing environmental or mechanical protection of the underlyingarticle, upon application onto said article. The protective formulationmay optionally comprise photostabilizers, such as UV absorbers, hinderedamine light stabilizers (HALS), antioxidants, singlet oxygen scavengers,etc.

A “stabilizing additive” is a material added to a formulation comprisingsolid particles, or a dispersion, to uniformly distribute, preventagglomeration, and/or prevent settling of solid materials in saiddispersion in said liquid carrier medium to result in an enhancement ofluminous intensity. Such stabilizing additives generally comprisedispersants and/or rheology modifiers.

A “preformed article” is any article onto which at least one layer whichis photoluminescent or otherwise may be applied. A preformed article maybe rigid or flexible.

A “film” is a thin skin or membrane that can be rigid or flexible. Anexample of this is the layer resulting from the application of aformulation and drying it. One or more layers can then constitute afilm.

As used herein, “visible electromagnetic radiation” is characterized byelectromagnetic radiation with wavelengths in the region between about400 nanometers (“nm”) and about 700 nm.

“Photolytic degradation” is deterioration or change in properties, suchas observed color or luminescence characteristics, that is induced byelectromagnetic radiation.

“Thermal degradation” is deterioration or change in properties that isinduced by heat.

“Sustained emission” is emission of electromagnetic radiation at desiredwavelengths for prolonged periods of time. Prolonged period of timeduration has meaning within the context of the usage. For example, inoutdoor usage with primary emission source embodied within the filmelement, this may mean one to several years of outdoor usage.Alternatively, for applications in solid state LEDs for convertingprimary emission to secondary emission of different colors, includingwhite light, sustained emission implies lifetimes greater than 10,000hours.

LED Lifetimes: The lifetime of light emitting diodes is defined by acombination of catastrophic failure rate and failure due to loss oflumen maintenance. For example a B10 value indicates the time by which10% of a population has failed. Similarly, a failure criterion of L70classifies as a failure any unit that has lost more than 30% of itslight output. Therefore, a (B10, L70) lifetime is defined as that timeby which 10% of a population has either failed catastrophically or haslost at least 30% of its light output.

An “excimer” is a short lived dimeric or heterodimeric molecule formedfrom two species at least one of which is in an electronic excitedstate.

The “glass transition temperature” is the temperature that identifiesthe transition of a polymer material from a rubbery state to a glass,and is characterized by a discontinuity in the variation of specificvolume with temperature.

A “lenticule” as used herein, is one of a number of corrugations orgrooves molded or embossed into a surface.

A “lenticular” lens is a lens made of an array of lenticules. A“lenticular film,” as used herein, is a film having a surface on which alenticular lens is formed.

A “prismatic lens,” as used herein, is a a lens incorporating aprescribed prism. A “microprismatic film,” as used herein, is a filmhaving an array of small prismatic lenses formed on a surface of thefilm. A microprismatic film can be, in one exemplary embodiment, but isnot restricted to this exemplary embodiment, an array of micro Fresnellenses.

The following disclosure describes the methods and the materials forcreating stable photoluminescent multilayer electromagnetic energyconversion elements. These elements possess a number of superiorqualities, such as ability to produce sustained emission, high energyconversion efficiency, minimized back scatter properties, energyconversion flexibility for tailoring secondary emission, and physicaland chemical stability to environmental exposure.

In one aspect, the present teachings provide for a method for convertinga primary electromagnetic radiation, or pumping radiation, into another,secondary electromagnetic radiation having a different spectrum,generally characterized by a higher average wavelength. The method ofenergy conversion of the present teachings is described with referenceto FIG. 1. The source of excitation, can be electromagnetic radiationsuch as daylight or other ambient light, or electrical or chemicalenergy, and is represented in FIG. 1 with a solid upward arrow. Forexample, FIG. 1 can be understood to represent a case in which thepumping radiation E1 is the emission from a persistent phosphor that hasbeen previously excited by external illumination into absorption bandA1. FIG. 1 can also represent the case in which the pumping radiation E1is provided by a chemiluminescent source, where A1 can be interpreted asthe chemically generated excited state. FIG. 1 can also represent thecase in which the pumping radiation E1 is provided by anelectroluminescent source, where A1 can be interpreted as the emissivestate in the electroluminescent device.

The primary radiation or pumping energy, which originates from theexcitation source may be provided by a radiation source that is embodiedwithin the energy conversion element, such as a persistent phosphor, asis shown in FIG. 2B (Layer 12), that has been excited with sunlight, orwith other external illumination. Alternatively, the pumping energy maybe provided by a radiation source that is external to the energyconversion element, as is shown in FIG. 2A, such as a solid statedevice, for example an LED, a laser, or an electroluminescent element,etc. It should be noted that embodiments wherein the energy conversionelement is within the solid state device, as well as embodiments whereinthe radiation source is in proximity to the energy conversion elementare within the scope of these teachings. In such alternative case, theexcitation may arise from electrical energy that stimulates emissionfrom the solid state device, or chemically, as in a chemiluminescentdevice.

With continued reference to FIG. 1, the electromagnetic energy spectrumof the principal electromagnetic radiation is converted by the method ofthe present teachings into a new radiation, having a spectrum generallyof a higher average wavelength, through a cascade of absorption/emissionevents by one or a set of photoluminescent materials, for exampleorganic fluorescent dyes. Each individual photoluminescent material ischaracterized by a radiation energy absorption spectrum Ai, a radiationenergy emission spectrum Ei, and a characteristic time constant Tibetween radiation absorption and radiation emission (where i=1, 2, 3 . .. ). Preferably, some or all of the new electromagnetic radiationproduced by the method of the present teachings is visibleelectromagnetic radiation.

Further referring to FIG. 1, in one aspect of the method of energyconversion of primary radiation in these teachings, essentially all ofthe primary electromagnetic radiation or pumping energy (characterizedas E1 in FIG. 1) is absorbed by a first photoluminescent materialcharacterized by energy absorption spectrum A2, energy emission spectrumE2 and a characteristic time constant T2 between energy absorption andenergy emission. Generally, the average wavelength of radiation emissionspectrum E2 is higher than the average wavelength of radiationabsorption spectrum A2. This difference in wavelengths is referred to asthe Stokes shift, and the energy corresponding to this difference inwavelengths is referred to as Stokes loss. The emission of the firstphotoluminescent material in the visible electromagnetic radiationspectrum can be allowed to escape, representing a new color which is notcharacteristic of the principal electromagnetic radiation.

Still referring to FIG. 1, in another aspect of the method for energyconversion of primary radiation in these teachings, a secondphotoluminescent material can be used to absorb the radiation emitted bythe first material, that is, E2. The second photoluminescent material ischaracterized by energy absorption spectrum A3, energy emission spectrumE3 and a characteristic time constant T3 between energy absorption andenergy emission. The radiation absorbed by the second materialrepresents some or all of the radiation emitted by the first material.The second material emits radiation and exhibits a Stokes shift to a yethigher wavelength than the first material. Additional photoluminescentmaterials having appropriate Stokes shifts can be chosen to furtherconvert the radiation until the desired emission wavelength is reached.These additional photoluminescent materials are characterized byradiation absorption spectra A4, A5, etc., radiation emission spectra,E4, A5, etc., and characteristic time constants between radiationabsorption and radiation emission T4, T5, etc. In this manner, aprincipal electromagnetic radiation characterized by a blue color, forexample, can be used to generate green, yellow, orange, or red light.Although, the film-based energy conversion elements can bephosphorescent or fluorescent photoluminescent materials, it ispreferred in some applications to use fluorescent photoluminescentmaterials, such as organic fluorescent dyes, as the energy conversionmaterials.

While reabsorption of emitted radiation can be an effective mechanismfor energy down-conversion, the transfer of energy does not requireemission and reabsorption. Alternatively, the transfer of energy inenergy down-conversion can occur through a Förster transfer mechanism,as illustrated in FIG. 11. With reference to FIG. 11, S₁ ^(g) representsthe state of a first photoluminescent material before absorbing light,S₁ ^(e) represents the state of the first photoluminescent materialafter absorbing light (the excited state), and A₁ represents the energyof the photon of light absorbed by the first photoluminescent material.Likewise, S₂ ^(g) represents the non-excited state of a secondphotoluminescent material, S₂ ^(e) represents the excited state of thesecond photoluminescent material, and E₂ represents the emission oflight from the second photoluminescent material corresponding to thetransition from its excited state back to its non-excited state. Withcontinued reference to FIG. 11, the second photoluminescent material canbe excited from S₂ ^(g) to S₂ ^(e) by transfer of energy from S₁ ^(e)without the emission of luminescent energy from the firstphotoluminescent material. As a result, the emission E₂ is produced fromdirect excitation of the first photoluminescent material. Förstertransfer can occur where the electronic characteristics of the emissionof a first photoluminescent material and the absorption of a secondphotoluminescent material are properly chosen, such that the transfer ofelectronic energy can occur by dipolar coupling without requiring theemission of a photon by the first photoluminescent material. Förstertransfer requires that the photoluminescent materials undergoing thetransfer of the electronic energy be close enough to experience theirrespective dipolar fields. As a result, Förster transfer requires asignificantly higher concentration of the second photoluminescentmaterial than is conventionally used for energy cascade.

In another aspect of the method of energy conversion of primaryradiation of the present teachings, only a portion of the principalelectromagnetic radiation is converted by the first material inaccordance with the foregoing disclosure to a radiation of a longerwavelength. All or a portion of the radiation that is emitted by thefirst material in combination with the remaining principal radiationwill create the desired final emission. In yet another aspect, all or aportion of the first materials' radiation emission is similarlyconverted by the second material to produce a radiation emission of astill longer wavelength. In the foregoing manner, a broad spectrum ofradiation emission can be attained, such that radiation emission ofwhite color is observed, for example.

Another aspect of these teachings provide for an energy conversionstructure rendered as a cast, coated, extruded film, generallycomprising multiple layers, or single or multiple layer particles. Theenergy conversion film structure is described with reference to FIG. 2Aand FIG. 2B.

Energy Conversion Layer: One of the layers of the structure provides thefunction of an energy conversion layer, which comprises one or morephotoluminescent materials that enable converting a primaryelectromagnetic radiation into another electromagnetic radiation havinga different spectrum, generally characterized by a higher averagewavelength. The energy conversion elements can be phosphorescent orfluorescent photoluminescent materials, with the fluorescent energyconversion materials, such as organic fluorescent dyes, being preferredin some applications. The energy conversion layer comprises one or morephotoluminescent materials which enable generating electromagneticemission energy of one or more new wavelengths. Referring to FIG. 2A,the energy of the primary electromagnetic radiation, or pumpingradiation, which is emitted by a radiation source that is external tothe structure, which is represented with a solid upward arrow, isconverted inside energy converting layer (2). In one aspect, the energyconversion layer (2) comprises a single layer incorporating one or a setof energy converting materials. In another aspect of the energyconversion layer, which may be preferred in some applications, theenergy converting layer (2) comprises several energy converting layersof different composition, each composition comprising one or a subset ofmaterials. The photoluminescent materials of the energy conversion layer(2) preferentially comprise organic fluorescent dye molecules.

In yet another aspect of the energy conversion structure of the presentteachings illustrated in FIG. 2B, wherein the primary radiation source,such as a high persistence phosphor, is also embodied within themultilayer structure, such as, for example, within the energy convertinglayer (12), or in a separate layer (12*, not shown) below the energyconversion layer. According to the aspect illustrated in FIG. 2B,wherein the pumping radiation emanates from the same side of thestructure as the desired emission, the photoluminescent materials mustbe chosen so as to permit sufficient optical transmission of theexcitation radiation (excitation radiation that causes emission of theprimary radiation) to initiate the pumping radiation. Thephotoluminescent materials of the energy conversion layer (12),preferentially comprise organic fluorescent dye molecules.

In another aspect of the energy conversion layer, the energy conversionlayer may additionally contain materials to optically scatter thepumping radiation and/or emitted radiation. The scattering of thepumping radiation serves to increase the effective optical path lengthof the layer thereby increasing the amount of pumping radiation absorbedand converted. The scattering of the emitted radiation serves to alterthe path of emitted rays that would otherwise be emitted from the edgesof the energy conversion layer due to total internal reflection.

In another aspect of the formation of the energy conversion layer, onecan prepare single or multilayered particles of the dye/polymer solidstate solution (as shown in FIG. 3 and further disclosed below) andprepare an energy conversion layer by dispersing the particles in eitherthe same or different polymer solution and subsequently rendering themas a layer on a suitable substrate. By preparing the dye/polymer solidstate solution as a particle the choice of polymer useful to obtain asolid state solution can be separated from the choice of polymer andother materials needed to prepare an appropriate layer structure.

Stability Enhancement Layer: The energy conversion structures of theseteachings, in addition to the energy conversion layer, also comprise oneor more stability enhancing layers (6), as shown in FIGS. 2A & 2B, whichprotect said photoluminescent materials of the energy converting layer(2) or (12) from light-induced (photolytic) degradation as well asthermal degradation so as to provide sustained emissions. In one aspectof creating the energy conversion structure, the stability enhancementlayer can be rendered as a discrete layer. While this is preferable, itshould be recognized that some functionality of the stabilityenhancement layer can also be achieved within the energy conversionlayer itself by suitable selection of the polymer matrix of the energyconversion layer. In certain applications it is advantageous to have thestability enhancement layer on both the top and bottom surfaces of theenergy conversion layer.

In another aspect of creating the energy conversion structure, one ormore energy conversion layers and one or more stability enhancementlayers can be rendered as discrete multilayer particles. It should berecognized that such discrete particles may be used as is for someapplications, or they may be subsequently incorporated into polymerformulations for the purpose of generating non-discrete (contiguous),planar or otherwise structures. For such a case, one can further createthe final structure for use as a single layer film by dispersing themultilayer particles in a suitable polymer solution and forming a layeron a suitable substrate.

Reflection Layer: With continuing reference to FIG. 2A and FIG. 2B, theenergy conversion structures of the present teachings may optionallycomprise a reflection layer (4) for forward-directing theelectromagnetic radiation emitted in a backward propagating direction.The reflection layer (4) allows for maximizing the collection andutilization of the radiation emitted by the energy converting layer (2).With reference to FIG. 2A, when the desired emission is intended to exitthe structure from a second surface, different from the first surfacefrom which the pumping radiation enters the energy conversion layer, thereflection layer may be a wavelength-selective reflector that issufficiently transmissive of the pumping radiation and reflective of theradiation emitted by the energy conversion layer. For the caserepresented by FIG. 2B, since the pumping radiation source is either inthe energy conversion layer, or a layer below, the reflection layer needbe reflective of the primary and converted radiation.

A reflection layer may also be incorporated into particle structuressuch as have been described in the foregoing disclosure. In one aspect,one or more energy conversion layers and stability enhancement layerscan be prepared on the surfaces of diffuse dielectric particles such astitanium dioxide. In another aspect, a wavelength-selective reflectionlayer can be prepared on the surface of a dielectric core, such ashollow glass spheres. One or more energy conversion layers and stabilityenhancement layers can subsequently be formed on the surfaces of suchreflection layer cores.

Diffusion Layer: Referring to FIG. 2A, the energy conversion structuresof the present teachings may optionally comprise a diffusion layer (5)to increase optical scattering. The diffusion layer (5) is opticallycoupled to the energy conversion layer, to increase the effectiveoptical path length of the energy conversion layer, thereby increasingthe amount of pumping radiation absorbed and converted by the energyconversion layer. In another aspect, the diffusion layer can beoptically coupled to the energy conversion layer to provide scatteringof the emitted radiation of the energy conversion layer, therebyreducing the amount of light that would be trapped within the energyconversion structure due to total internal reflection. In certainapplications, it is advantageous for the diffusion layer to be used inconjunction with a reflection layer such that the backscattered lightfrom the diffusion layer is redirected through the preferred surface.

Protective Layer: The energy conversion structure of the presentteachings may also optionally comprise a top protective layer (8) whichprotects the film structure from physical and chemical damage uponenvironmental exposure. This layer may also include other additives,such as photo-stabilizers, anti-oxidants, hindered amine lightstabilizers (HALS), singlet oxygen scavengers, etc.

Detailed Description of Layers

Energy Conversion Elements within Energy Conversion Layer: Thephotoluminescent materials used in the energy conversion layer of theenergy conversion structure of the present teachings are selected basedon their absorption and emission properties, with preference given tomaterials with high quantum yields. Preferably, the energy conversionlayer comprises organic fluorescent dyes. These dyes include, but arenot limited to, rylenes, xanthenes, porphyrins, phthalocyanines, andothers with high quantum yield properties. Rylene dyes are particularlyuseful. Rylene dyes include, but are not limited to, perylene ester anddiimide materials, such as 3-cyanoperylene-9,10-dicarboxylic acid2′,6′-diiosopropylanilide, 3,4,9,10-perylene tetracarboxylic acidbis(2,6-diisopropyl)anilide and1,6,7,12-tetraphenoxy-N,N′-di(2,6-diisopropylphenyl)-3,4:9,10-perylenediimidefor example. Xanthene dyes include, but are not limited to, Rhodamine B,Eosin Y, and fluorescein. Porphyrins include, for example,5,10,15,20-tetraphenyl-21H,23H-tetraphenylporphine and2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine.

Energy Conversion Layer—Dye Polymer Solid State Solution: The matrixinto which the dyes are dispersed can comprise of polymers or glasses.Polymers are particularly useful due to the greater range of availablematerials from which to sub-select so as to form a homogeneous mixtureof the energy conversion element and the polymer. Acceptable polymersinclude acrylates, polyurethanes, polycarbonates, polyvinyl chlorides,silicone resins, and other common polymers. Materials with glasstransition temperatures above the normal operating temperature of thematerial are particularly useful. The polymer matrix must be capable ofpreventing aggregation of the dye, that is creating a homogeneousmixture of the dye and the polymer, or a solid state solution of the dyeand polymer. Dye aggregation is one of the possible causes of loss offluorescence efficiency and in certain cases can also contribute tophotolytic degradation. If the dye molecules are allowed to formaggregates and microcrystalline forms, instead of a solid statesolution, the dye molecules excited by electromagnetic radiation canundergo static self-quenching, effectively lowering the yield of dyefluorescence, and therefore lowering the efficiency of energyconversion. In some dyes, high dye concentrations can lead to enhancedphoto-degradation, through efficient energy migration to chemicallyactive traps or through self-sensitized photo-oxidation stimulated bytriplet states accessed primarily from excimers.

When dye aggregation leads to the creation of crystals in a condensedmedium, the effective molar extinction of the dye declines, requiringstill higher concentrations to maintain an acceptable optical density.In addition, the characteristic size of microcrystalline forms canapproach wavelengths of visible electromagnetic radiation, leading tolight scattering, effectively increasing the path length that the lightmust travel before escaping from the film structure. In instances wheredye concentrations are of high optical density, the light scattering canlead to increased reabsorption of fluorescence, resulting in a reductionin the energy conversion efficiency.

It should be noted that it is very difficult, if not impossible, todirectly measure/determine if the chosen dye is in solid state solutionin the polymer. The determination of this is inferential and resultsfrom examining absorption and emission spectra. While molecular dimersdo not usually show a change in the electronic absorbance spectrum, theeffects of excimer formation can be observed as significant broadeningof the emission spectrum, a shift to longer wavelengths, and a reductionin total emission in the fluorescence emission spectrum. Largermicrocrystalline aggregates manifest changes in the radiation absorptionspectrum. For example, J-aggregates show a red shift in the absorptionspectrum (to higher wavelength), while H-aggregates show a shift to theblue.

In the construction of an effective energy conversion layer capable ofsustained emissions, as mentioned above, it is important to achieve ahomogeneous mixture of dye and polymer in the solid state, that iswithout the formation of aggregates. One of the ways that such ahomogeneous mixture may be prepared is by dissolving the polymer and thedye in a suitable solvent. The selection of the matrix polymer, solvent,and other dispersants and additives, has to be such that the dye remainsdissolved throughout the coating and drying process. The state of thedye in the polymer in the solid state should be essentially free ofaggregates, that is the dye must be in a solid state solution in thepolymer after drying. The dye may also be incorporated into the polymerwithout the use of solvents by heating a mixture of the polymer and dyeto the melt state with physical mixing. Care must be taken during such aprocess to assure that the dye is completely dissolved, while limitingthe mixing time and temperature to minimize polymer and dye thermaldegradation. It should be noted that if the dye and polymer are notadequately compatible, a homogeneous mixture may not result in spite oftaking the precautions described above. The polymer should also beselected for its thermal stability at the operating temperaturesexperienced by the multilayer film structure. Insofar as polymer chainmotion affects the rate of polymer decomposition and depolymerization,materials with glass transition temperatures above the normal operatingtemperature of the film are preferred for many polymer classes. However,there are exceptions to this general principle. For example, siliconerubbers are known to possess exceptional thermal stability in spite ofhaving glass transition temperatures well below normal roomtemperatures.

Before one can determine whether a specific dye is in a solid statesolution in a specific chosen polymer, it is important to firstestablish a reference absorption and emission spectra for the specificenergy conversion material. This can be done by generating absorptionand emission spectra in a variety of solvents, and by analyzing theresulting spectra together with any published spectra, so as to select asuitable reference solvent, wherein the energy conversionphotoluminescent material is soluble in the solvent at the requiredconcentration. The reference spectra, however, is generally generated atvery dilute concentrations. The spectra thus generated can then serve asthe reference absorption and emission standard. When it is necessary toselect other solvents for solubility reasons, comparisons can be madewith reference spectra at the desired concentration, as well as bycomparing spectra of a very dilute solution of the same material, todetermine if the absorption of the energy conversion photoluminescentmaterial is shifted to much longer wavelengths, which is diagnostic ofmolecular aggregates, and the emission spectrum is neither quenched norbroadened and shifted to longer wavelengths, which is diagnostic ofexcimer formation. For materials that have been well researched, therewill be suitable reference spectra that can be used as a basis forrecognizing significant spectral shifts. For newer materials, one has torely on spectra generated at very dilute concentrations together withany absorption and emission as a reference basis, and significant shiftscould be interpreted as indicative of aggregate formation. Thesolubility can be checked with a light scattering apparatus, and theabsorption and emission by a fluorescence spectrophotometer. Theselected solvent should also be capable of dissolving the selectedpolymer. To determine if the selected dye will be in a solid statesolution in the polymer, different polymers can be similarly screened,together with the energy conversion photoluminescent material, by firstobserving the absorption and emission in a formulation comprising theenergy conversion photoluminescent material, the polymer, and thesolvent, and second, by observing the absorption and emission in acoating prepared from the formulation by removing the solvent andanalyzing comparisons with the reference spectra for evidence ofaggregate and/or excimer formation.

Energy Conversion Layer—Other Additives: Additional components may beadded to the formulation to facilitate the dissolution and coating ofthe energy conversion photoluminescent material and the polymer, such asdispersants, wetting agents, defoamers, rheology modifiers, and levelingagents. Dispersants, wetting agents, defoamers, and leveling agents mayeach be oligomeric, polymeric, or copolymeric materials or blendscontaining surface-active (surfactant) characteristic blocks, such as,for example, polyethers, polyols, or polyacids. Examples of dispersantsinclude acrylic acid-acrylamide polymers, or salts of amine functionalcompound and acid, hydroxyfunctional carboxylic acid esters with pigmentaffinity groups, and combinations thereof, for example DISPERBYK®-180,DISPERBYK®-181, DISPERBYK®-108, all from BYK-Chemie, and TEGO® Dispers710 from Degussa GmbH. Wetting agents are surfactant materials, and maybe selected from among polyether siloxane copolymers, for example, TEGO®Wet 270, non-ionic organic surfactants, for example TEGO® Wet 500, andcombinations thereof. Suitable rheology modifiers include polymeric ureaurethanes and modified ureas, for example, BYK® 410 and BYK® 411 fromBYK-Chemie®, and fumed silicas, such as CAB-0-SIL® M-5 and CAB-0-SIL®EH-5. Deaerators and defoamers may be organic modified polysiloxanes,for example, TEGO® Airex 900. Leveling agents may include polyacrylates,polysiloxanes, and polyether siloxanes. Quenchers of singlet molecularoxygen can also be added, such as 2,2,6,6-tetramethyl-4-piperidone and1,4-diazabicyclo[2.2.2]octane. Each such material must be tested toassure that it does not cause aggregation of the energy conversionphotoluminescent material, and that the material does not react, eitherthermally or photochemically, with the energy conversionphotoluminescent materials.

According to the present teachings, one preferred photoluminescentcomposition, includes from about 25%-45% of binder resin, about 50%-70%of liquid carrier, 0%-2% dispersing agent, 0%-2% rheology modifyingagent, 0%-2% photostabilizer, 0%-2% de-aerating agent, 0%-2% wettingagent, and 0.01%-0.2% photoluminescent fluorescent material.

Energy Conversion Layer—Preparation Methods: Once proper materials andconcentrations have been identified, a variety of methods can be used toprepare an effective energy conversion layer. Such methods includecoating a layer that is generally planar on a support, the layer beingprepared from a formulation in a liquid carrier medium. For example,such coatings can be deposited by painting, screen printing, spraying,slot coating, dip coating, roller coating and bar coating. In addition,an effective energy conversion layer may be prepared by methods that donot use a liquid carrier medium. For example, a solid solution of anenergy conversion photoluminescent material in a polymer can beconverted to an energy conversion layer by extrusion, injection molding,compression molding, calendaring, and thermoforming. It should be notedthat some of these methods can be particularly useful in producingnon-planar layers. The energy conversion layer can also be prepared byforming particles by any one of a number of methods of preparing suchparticles. When the energy conversion element is prepared as acombination of several different energy conversion layers, theindividual layers can be sequentially coated, or the individual layerscan be separately prepared and later laminated or embossed together toform an integral layer. Alternatively, an integrated energy conversionlayer can be prepared by coextrusion of the individual layers. In allcases in which the energy conversion layer is prepared by combiningseveral different energy conversion layers, the integrated energyconversion layer will only function properly if the separate layers areproperly ordered. That is, photoluminescent materials that produce ahigher average wavelength should generally be placed closer to theoutermost surface of the structure.

The elements of the structure of the present teachings may be combinedinto an integrated structure by a number of methods well known in theart. For example, the energy conversion layer may be coated onto thereflective layer, or it may be separately prepared and subsequentlylaminated to the reflective layer. An integrated structure may beconstructed by sequential coating or printing of each layer, or bysequential lamination or embossing. Alternatively, several layers may becombined by sequential coating, lamination or embossing to form asubstructure, and the required substructure then laminated or embossedtogether to form the integrated multilayer structure. As had beenmentioned above, the integrated structure can also be created by formingmultilayer particles comprising energy conversion layer, or, comprisingenergy conversion layer and stability enhancement layer and subsequentlyrendering them as a planar layer.

Stability Enhancement Layer—Details: Although photolysis and thermolysisphenomena have been studied, such studies at best only serve asguidelines for identifying the variables that may affect the performanceof a photoluminescent material in a given situation. Usually, acombination of factors is responsible for causing photolytic or thermaldegradation and not all variables are equally important. Hence, a greatdeal of systematic experimentation is required to determine a suitablecomposition and structure for stabilizing the photoluminescent materialagainst photolytic and thermal degradation. A first objective indetermining a suitable structure is to screen a variety of polymersolvent systems, wherein both the photoluminescent material and thepolymer are soluble in the solvent, preparing a film from such aformulation, and studying its photolytic and thermal degradation. Toqualify a polymer for its thermal properties, the polymer should bethermally stable. Generally, it is advantageous to select a polymerwhich glass transition temperature is higher than the expected operatingtemperature. It would be desirable to select a polymer with a glasstransition temperature that is 10° C. to 15° C. higher than theoperating temperature. In studying the thermal stability of the polymer,in many cases, color change upon prolonged periods of exposure to heatis evidence of thermal degradation. After thermal qualification, thepolymer is tested for photolytic stability. Following thisqualification, the dye/polymer system is similarly tested for thermaland photochemical stability. In most cases, loss of optical density ofthe dye is the primary criterion of system stability. In addition,changes in fluorescence should also be monitored as an indication ofdegradation.

By way of example, the following experimentation was undertaken for acase wherein the multilayer film structure was to operate outdoors in anenvironment wherein the operating temperature of the film conversionelement was anticipated to be 50° C. or lower. The polymer and thephotoluminescent material solvent system were screened by testing in anenvironmental test chamber for the purposes of selecting the appropriatepolymer/solvent system that was photolytically and thermally stable. Forpurposes of illustration, Table 1 demonstrates the effect that choice ofpolymer and solvent can have on dye stability, using acrylic polymers(Neocryl® materials) and polyurethanes (Solucote® materials) asexamples. The dye/polymer combinations were screened without the use ofany additional stabilization structures using test conditions presentedin Table 1. This is because for screening it was important to selecttest conditions that would allow discrimination between species quickly.As is apparent from Table 1, different polymer/solvent systems can havea remarkable effect on photolytic and thermal degradation. The specificdye used in this example exhibits low photolytic stability in urethanematerials as compared to acrylics. Materials from the same polymer classcan also manifest different photolytic stability characteristics. Thisis because the different manufacturing processes utilized by variousmanufacturers can result in varying amounts of residual monomers,residual level and type of catalysts, etc., creating variables affectingpolymer stability characteristics and, in turn, impacting the stabilityof the energy conversion materials.

Once a suitable polymer/solvent system for the energy conversionmaterial under consideration was determined, the impact of variousadditives, such as UV absorbers, hindered amine light stabilizers(HALS), to reduce photolytic and thermal degradation was explored. Anexample of this line of experimentation is presented in Table 2. Forthis dye and polymer combination, it was found that UV absorbers offeredsignificant photostability improvement, while free radical quenchers andhindered amine stabilizers had little effect. Having selected a baselinesystem comprised of polymer, solvent, additives (UV absorbers,antioxidants, etc.), it was concluded that it would be desirable to lookfor additional factors that could retard photolytic degradation.

TABLE 1 Photolysis^(1,2) Time (hours) Dye Binder³ Solvent 6 24 54 723-cyanoperylene- Neocryl ® B805 Toluene −3.0% −10.2% −19.3% −25.5%9,10-dicarboxylic acrylic Toluene/dioxolane −5.7% −14.7% −17.1% −24.8%acid 2′,6′- Dimethyl −1.1% −5.6% −10.8% −15.8% diisopropylanilideCarbonate Ethyl Acetate −2.6% −9.2% −14.9% −21.3% Acetone −4.7% −17.0%−33.8% −38.1% Neocryl ® B805 Toluene/MPA −0.9% −2.6% −5.2% −7.7%acrylic/CAB Neocryl ® B818 none −3.7% −24.2% −49.6% −62.0% acrylicSolucote ® 148-102 DMF −3.0% −21.8% −58.1% −73.4% Solucote ® 141-301 DMF— −50.7% −79.8% −88.7% 12.7% Solucote ® 12875 DMF — −62.7% −71.0% *27.1% ¹Photolysis performed with a water-cooled, high pressure xenon arclamp filtered to simulate a daylight spectrum to deliver 0.76 W/m² at340 nm. ²Data are presented as a percentage loss in absorbance (opticaldensity). ³Neocryl ® B805, Neocryl ® B818, and all Solucote ® materialswere obtained from DSM Neoresins; cellulose acetate butyrate (CAB) wasobtained from Eastman Organic Chemicals. ⁴Empty cells indicate that atest was stopped due to a significant loss in optical density.

TABLE 2 Photolysis^(1,2) Time (hours) Dye Binder³ Stabilizer 6 24 54 723,4,9,10-perylene Neocryl ® none −19.8% −34.9% −47.9% −60.2%tetracarboxylic B818 acrylic 2% Tinuvin ® 123 −13.1% −25.0% −40.5%−51.1% acid bis(2,6- 2% Irganox 1035 −16.8% −46.2% −66.6% −75.7%diisopropyl) 2% Tinuvin ® 405 −3.30% −5.96% −12.94% −21.95% anilide 2%Cysorb UV-531 −8.5% −14.5% −19.3% −45.3% 2% Chimmasorb 944 −13.0% −37.9%−60.7% −85.7% Neocryl ® none −3.6% −13.5% −28.8% −23.9% B805acrylic 2%Tinuvin ® 405 −1.7% −4.0% −6.6% −9.4% Tol/EtAc w/EGME Lucite 47G none−3.1% −6.1% −15.5% −17.6% 2% Tinuvin ® 405 −1.5% −3.0% −6.0% −8.9%Solucote ® none −40.5% −87.3% −94.0% −94.9% 148-102 2% Tinuvin ® 405−28.0% −76.7% −91.2% −92.9% 2% Tinuvin ® 123 −38.5% −83.8% −93.4% −94.6%2% Irganox ® 1035 −28.6% −80.1% −93.1% −93.7% 2% Tinuvin ® 405 −17.4%−73.9% −90.9% −92.8% ¹Photolysis performed with a water-cooled, highpressure xenon arc lamp filtered to simulate a daylight spectrum todeliver 0.76 W/m² at 340 nm. ²Data, are presented as percentage loss inabsorbance (optical density). ³Tinuvin ® 123, Tinuvin ® 405, Irganox ®1035. and Chimmasorb ® 944 were obtained from Ciba; Cyasorb UV-531 wasobtained from CYTEC Industries.

Need for Stability Enhancement Layer: Many organic dyes suffer fromphotolytic decomposition either as solutions in a solvent or as ahomogeneous mixture with a polymer when irradiated in the presence ofair. In some cases, the decomposition can be traced to the decompositionof a surrounding polymeric matrix, which can generate free radicalintermediates that attack the dye molecules. Some polymers contain UVchromophores that, after excitation, can sensitize the formation ofsinglet oxygen. In some cases, the dye can sensitize its owndecomposition through the generation of singlet oxygen. Whilefluorescent dyes generate very few triplet species in theirphotophysics, the small amount that occurs can effectively transferenergy to available molecular oxygen to generate reactive singletoxygen. This species can then attack nearby dye molecules to generatenon-fluorescent products. In addition, the choice of polymer matrix canaffect the lifetime of singlet oxygen. Citations have been made aboutthe beneficial effects of the presence of oxygen. For example, in“Photodegradation of Polymer-Dispersed Perylene Di-imide Dyes,” byNobuaki Tanaka, et al., Applied Optics, Vol 45 (2006), pp. 3846-51, itis reported that dyes can also degrade by photoreduction mechanisms, inwhich case available oxygen can both compete for the reductant, as wellas reoxidize the reduced dye. The authors identify that the presence ofoxygen in the vicinity of the dye can reduce the rate of photolyticdegradation. In other words, preventing the transmission of oxygen canlead to more rapid dye degradation. Surprisingly, the opposite is foundto be true. Specifically, it has been determined that certain polymercoatings when made from materials that are known to significantly retardthe diffusion of oxygen have a dramatic impact on improving photolyticstability. Thus, to extend the stability of the energy conversionmaterials, a stability enhancement layer is incorporated within themultilayer structure of the present teachings. Useful materials that canbe used in the stability enhancement layer of the teachings include, butare not limited to, a number of materials commonly used today to inhibitthe transmission of air, especially in applications such as foodpackaging. Such useful materials include, but are not limited to,polyvinyl alcohol, ethylene vinyl alcohol copolymers, polyvinylchloride, polyvinylidene chloride copolymers (saran), nylons,acrylonitriles, polyethylene terephthalate polyester, polyethylenenaphthalate, polytrimethyl terephthalate, liquid crystal polymers,transparent inorganic oxide coatings, nanocomposites, oxygen scavengers,aromatic polyketones and any combinations or blends thereof.

It has been mentioned above that in certain situations it isadvantageous to have the stability enhancement layer on both the frontand bottom surfaces of the multilayer energy conversion structure. Oneof the preferred ways of achieving that is to create a stabilityenhancement layer that inhibits the transmission of oxygen on one sideof the energy conversion layer, for example on the side of the convertedemission emitting surface, and for the other side the dielectricreflection layer prepared from metal oxide coatings. The low diffusionof oxygen through the metal oxide layers serves as an effective secondstability enhancement layer in this case. It should also be noted thatfor certain applications, a suitably thick polyester substrate ontowhich are rendered the energy conversion layer and the stabilityenhancement layer can also provide some functionality in retarding thediffusion of oxygen on from the opposite side.

Singlet molecular oxygen is presumed to be an important reactive speciesin the photolytic degradation of dyes. While reducing the concentrationof oxygen is an effective deterrent to the creation of singlet oxygen,this species can also be quenched by a number of additives therebypreventing it from reacting with the dye. Such quenchers should beplaced in the layer in which the singlet oxygen is most readily formed,that is in the energy conversion layer. Examples of singlet oxygenquenchers include, but are not limited to,2,2,6,6-tetramethyl-4-piperidone, 1,4-diazabicyclo[2.2.2]octane, anddiphenylsulfide.

Reflection Layer—Details: For the energy conversion structure of thepresent teachings to function as an efficient primary radiationconversion device, the desired emission for the device should take placeon the hemisphere on the forward-propagating side of the principalemitter. Therefore, it is important to redirect the backward propagatinglight. Although use of photoluminescent materials, such as fluorescentdyes, that are in solid state solution in the polymer matrix minimizesback scatter by eliminating small aggregates, backward propagatingemission remains, since dyes are isotropic Lambertian emitters.Referring to FIG. 2A and FIG. 2B, reflection layer (4) redirects allwavelengths of the radiation into the forward-propagating hemisphere.Reflection layer (4) can be advantageously used behind the energyconversion layer (2) or (12), wherein the primary emitter is also aLambertian emitter. Reflection layer (4) can also be designed to allowtransmission of the pumping radiation when such radiation occurs in therearward-propagating hemisphere, as in FIG. 2A.

The preferred reflection layer is dependent on the design of theapplication. For example, when primary radiation is provided by a sourcesuch as an LED from behind the film structure, such as in FIG. 2A, awavelength-selective specular reflection layer is preferred so as not todisrupt the transmission of light to the energy conversion layer. Such aselective reflection layer can be made by alternately layeringnon-metallic materials with high and low dielectric constants with layerthicknesses of approximately ¼ the wavelength of the light to bereflected. The characteristics of the reflection layer are a function ofthe refractive index difference between the materials used in the lowdielectric layers and those in the high dielectric layers, as well asthe number of layers in the reflection layer structure. Typicalrefractive index differences fall in the range 0.05-1.0. Typical lowdielectric materials have refractive indices at visible wavelengths inthe range 1.35-1.50 and include materials such as, but not limited to,MgF₂, CaF₂, and fused silica. Typical high dielectric materials haverefractive indices that range from 1.45-2.5 and include, but are notlimited to, aluminum oxide, zirconium oxide, and titanium oxide. Thehigh and low dielectric materials may also comprise optical polymers ofsuitable refractive index differences. Such dielectric stacks can bedesigned to pass a selective band of wavelengths, or they may be createdas high- or low-pass filters.

On the other hand, when the primary radiation originates within themultilayer structure such that the primary emission generating materialsare within the energy conversion layer, or in a layer below such as inFIG. 2B, or wherein the primary emission originates from the front orviewing side of the multilayer structure a diffuse reflection layer thatis reflective of both the primary and secondary emissions is preferred.Such a diffuse reflection layer can be made as a film or coating orlayer from materials that scatter but do not absorb the wavelengths ofinterest. For example, white poly(ethylene terephthalate) can beprepared by extruding the polymer incorporated with titanium dioxide.Such materials are sold by, for example, Dupont Corporation under thetrade name Melinex®. Alternatively, a diffuse reflection layer may beprepared by fluid coating a dispersion of the material in a polymericbinder onto a reflective substrate. For example, reflective substratesinclude poly(ethylene terephthalate) onto which has been deposited athin aluminum coating. Suitable materials for diffuse reflection layersinclude titanium oxide, silicon dioxide, etc. The polymer is usuallychosen based on compatibility with other layers to which it will belaminated, so as to provide good adhesion and optical coupling.Acceptable polymers include, but are not limited to, acrylates,polyurethanes, polycarbonates, polyvinyl chlorides, silicone resins, andother common polymers. In particular, it is important that the polymermaterials do not absorb the pumping or emitted radiation. In addition,additives such as, but not limited to, dispersants, wetting agents,defoamers and leveling agents, may be added to the formulations used forcoating such materials to aid in the dispersion of the scatteringmaterials and the coating of the formulation. Such dispersants, wettingagents, defoamers, and leveling agents may each be oligomeric,polymeric, or copolymeric materials or blends containing surface-active(surfactant) characteristic blocks, such as, for example, polyethers,polyols, or polyacids.

Diffusion Layer—Details: In some aspects of the present teachings it canbe desirable to provide optical scattering of radiation. Opticalscattering is provided by materials separated into discrete domains thatpossess significantly different optical indices of refraction, in mostcases due to the materials having different densities. In addition, thesizes of the domains have a significant effect on the efficiency ofscattering, with larger domains providing more scattering. Scatteringlayers can be produced in a variety of polymers, including acrylics,polyesters, polyurethanes, and the like. The polymer for the diffusionlayer is usually chosen based on compatibility with other layers towhich it will be laminated, so as to provide good adhesion and opticalcoupling. Materials that can commonly be incorporated into polymerlayers to provide scattering include titanium dioxide, mica, and glassmicrospheres. In addition, small domains of incompatible polymers may beformulated to produce diffuse, scattering layers. In certainapplications, it is advantageous for the diffusion layer to be used inconjunction with a reflection layer, such that the backscattered lightin the diffusion layer is redirected through the preferred surface.

The diffusion layer can be prepared by extrusion of a polymer layer thatincludes the scattering materials. Alternatively, the diffusion layercan be prepared by coating a formulation of the scattering materialswith a polymer binder in a suitable solvent onto a clear substrate.Acceptable substrate materials include, but are not limited to,polyesters, polycarbonates, acrylics, polyurethanes, and the like. Thepolymer is generally chosen based on compatibility with other layers towhich it will be laminated, so as to provide good adhesion and opticalcoupling. Acceptable polymers include, but are not limited to,acrylates, polyurethanes, polycarbonates, polyvinyl chlorides, siliconeresins, and other common polymers. In particular, it is important thatthe polymer materials do not absorb the pumping or emitted radiation. Inaddition, additives such as, but not limited to, dispersants, wettingagents, defoamers and leveling agents, may be added to the formulationsused for coating such materials to aid in the dispersion of thescattering materials and the coating of the formulation. Suchdispersants, wetting agents, defoamers, and leveling agents may each beoligomeric, polymeric, or copolymeric materials or blends containingsurface-active (surfactant) characteristic blocks, such as, for example,polyethers, polyols, or polyacids.

Protective Layer—Details: The film structure of the present teachingsalso comprises a top protective layer which protects the film structurefrom physical and chemical damage upon environmental exposure. Since thematerial properties of common air barrier materials render themsensitive to normal physical and chemical wear and degradation, therobustness of the structure is improved by the addition of a topprotective layer to provide these properties. For example, whenpolyvinyl alcohol or other water compatible polymer materials areincorporated within the stability enhancement layer, it is preferred toposition a protective layer over the stability enhancement layer to helpshield the stability enhancement layer from atmospheric humidity. Usefulprotective materials for the film structure include, but are not limitedto, poly(methylmethacrylate), polycarbonate, and polyesters. Since manypolymers can degrade upon exposure to ultraviolet (UV) radiation, it ispreferred that this top layer also contains absorbers of UV light thatalso protects the polymer and dye layers below it. Acceptable UVabsorbers include, but are not limited to, hydroxybenzophenones, such asChimmasorb® 81 from Ciba, hydroxyphenyl benzotriazoles, such as Tinuvin®326 from Ciba, and hydroxyphenyl triazines, such as Tinuvin® 405 fromCiba. The top surface of the top protective layer may be modified tooptimize light extraction from the structure. Such modifications mayinclude, but are not limited to, antireflection layers or brightnessenhancement structures, such as those and similar to those described inU.S. Pat. Nos. 4,542,449, 4,791,540, 4,799,131, 4,883,341, 4,984,144,all which are incorporated by reference herein in their entirety. Inaddition, the top layer may contain additional materials to enhancestability referred to earlier as photostabilizers such as antioxidants,HALS, singlet oxygen scavengers, etc.

Methods of Providing Primary Emission: It should be recognized that thestructure of the present teachings is not limiting with regard to themethod of introducing the primary electromagnetic radiation. Forexample, as shown in FIG. 4, the primary electromagnetic radiation maybe supplied from an illumination source (16) through a waveguide (18),with the energy conversion layer (20) positioned at a region designed toextract light from the waveguide. In one aspect, the energy conversionlayer may be optically coupled to the waveguide, in which case the lightextraction mechanism may occur either within the energy converting layeritself or within a separate optical structure incorporated with thewaveguide. Where the energy conversion layer is optically coupled to thewaveguide, it is preferred to have a stability enhancement layer on thelight exiting side of the energy conversion layer. In another aspect,the energy conversion layer may be positioned without being opticallycoupled to the waveguide. In this aspect, the light extraction mechanismoccurs within a separate optical structure of the waveguide, andpreferably, the stability enhancement layer is on both the top andbottom surfaces of the energy conversion layer. Embodiments wherein thestability enhancement layer is absent are also within the scope of theseteachings.

If the waveguide has multiple regions from which light can be extracted,different energy conversion structures with different conversionproperties can be positioned at each such region to provide, forexample, multiple colors from a single source. Multiple energyconversion structures can be combined with multiple sources to provide amulticolor display, as illustrated in FIG. 5. In addition, the energyconversion structure is not required to have a planar geometry. Forexample, a cylindrical structure (22) may be used with a cylindricalsource (24), such as a fluorescent or chemiluminescent tube, asillustrated in FIG. 6. It should be noted, as stated above, that theabove examples, applications and embodiments are not limitations ofthese teachings and other embodiments in which the primaryelectromagnetic radiation source is in varying degrees of proximity tothe energy conversion structures, including embodiments in which theenergy conversion structures integral to the electromagnetic radiationsource, are within the scope of these teachings.

EXEMPLIFICATIONS

The present teachings, having been generally described, will be morereadily understood by reference to the following examples, which areincluded merely for purposes of illustration of certain aspects andembodiments of the present teachings, and are not intended to limit thescope of these teachings.

Example 1 Preparation of Energy Conversion Layer (Characterized as 2 inFIG. 2A)—Film A

A formulation containing 48 parts of acrylic copolymer, such as Neocryl®B805, 72 parts toluene, 0.125 parts wetting agent, such as NoresilS-900, and 0.626 parts defoamer, such as Foamex N, was prepared andstirred at room temperature. To this solution was added a solution of0.086 parts 3-cyanoperylene-9,10-dicarboxylic acid2′,6′-diiosopropylanilide in 4.32 parts dioxolane, and the mixture wasstirred until fully dissolved. The formulation was coated on releasebase at 10 mils or 250 microns wet thickness and dried at 35° C. for 4hours and then at 80° C. for an additional 12 hours (for ensuring lowresidual solvent) to yield a 3 mil or 75 micron thick, yellow daylightcolor film energy conversion layer.

Example 2 Preparation of Stability Enhancement Layer (Characterized as 6in FIG. 2A and FIG. 2B)—Film C

A formulation containing 20 parts poly(vinyl alcohol) and 230 partswater was prepared and mixed at room temperature, and then heated to 95°C. and slowly stirred for two hours until the solution was clear. Theformulation was cooled to room temperature and coated on release base at10 mils wet thickness, dried at 35° C. for 4 hours and then at 80° C.for 12 hours to yield a 1 mil or 25 microns thick, clear, colorlessfilm.

Example 3 Preparation of Protective Layer (Physical & ChemicalProtection Together with UV Protection and Other Stabilizing Additives)(Characterized as 8 in FIG. 2 a and FIG. 2 b)—Film B

To 403 parts toluene, 1440 grams 4-chlorobenzotrifluoride, and 820 partsmethyl acetate was added 28 parts of UV absorber, such as Tinuvin® 405,and the solution was stirred. To the solution at room temperature wasadded 1440 parts of acrylate copolymer, such as Neocryl® B805, 173 partsplasticizer, such as Plasthall P-670, 4.3 parts wetting agent, such asNoresil S-900, and 17.3 parts defoamer, such as BYK-085, and the mixturewas stirred until the components were fully dissolved. The formulationwas coated on a release base at 5 mil wet thickness, and the coatingsdried at 35° C. for 12 hours to yield a 2 mil or 50 microns thick,clear, colorless film.

Example 4 Impact of Multilayer Stabilized Energy Conversion Element byComparison to Non-Stabilized Structures: LED Provides Primary RadiationSource

-   -   Laminate 1: Film A was heat laminated at 266° F. to a 10 mil        clear polyethylene terephthalate (PET) polyester carrier and the        release base was removed to yield an energy conversion element        without a stability enhancement layer or a protective layer.    -   Laminate 2: A portion of Film B was heat laminated to a portion        of Laminate 1 at 266° F. to yield an energy conversion element        with a protective layer comprising UV protection.    -   Laminate 3: To a second portion of Laminate 1 was heat laminated        a portion of Film C using a thermal adhesive. A second portion        of Film B was laminated to the top (PVA) surface, to yield an        energy conversion element with a stability enhancement layer and        a protective layer comprising UV absorbers.

Portions of Laminate 2, and Laminate 3 were subjected to continuousexposure with a deep blue LED (452 nm, 3.6 V, 490 mA) at a distance of0.5 cm to deliver approximately 420 mW/cm² to the structure, andmonitored for changes in optical absorption of the dye. Opticalabsorption was followed with a UV/Vis spectrophotometer, and digitizedspectra were corrected for optical scatter of the samples. Theobservations are summarized in FIG. 7. This level of total photonicenergy was selected so as to provide an accelerated test for anLED-pumped application. It can be seen that even though the stabilityenhancement layer is only on one side, there is a significant reductionin the photolytic degradation rate. Without the stability enhancementlayer, there is a density loss of greater than 90% in 700 hours,whereas, with the stability layer the density loss is only approximately10%. It should be noted that when density loss is translated toreduction in the fraction of primary radiation absorbed, the resultswith the stability enhancement layer are even more striking. With thestability enhancement layer the reduction in primary radiationabsorption is less than 2%.

Example 5 Impact of Stabilization Enhancement Layer on the Stability ofthe Energy Conversion Element by Comparison to Non-StabilizedStructures: Simulated Sunlight Exposure

-   -   Film D: An Energy Conversion Layer was similarly prepared as in        Example 1 except that the dye was        3,4,9,10-perylenetetracarboxylic acid        bis(2,6-diisopropyl)anilide and the polymer was a mixture of 20%        polyester plasticizer Plasthall 670 and 80% acrylic polymer        Elvacite 2014.    -   Laminate 4: Film D was heat laminated at 266° F. to a 3.8 mil        white polyester carrier and the release base was removed to        yield an energy conversion element without a stability        enhancement layer or a protective layer.    -   Laminate 5: To a portion of Laminate 4 was laminated a        pressure—sensitive adhesive, such as 3M Adhesive Transfer Tape        9626, and a layer of procured oxygen-barrier material, such as        Kuraray EF-E15 ethylene vinyl alcohol film to yield an energy        conversion element and a stability enhancement layer.        Portions of Laminate 4 and Laminate 5 were subjected to        continuous exposure in a sunlight simulator using a full        spectrum 1800 W xenon lamp filtered with a Daylight-Q optical        filter to deliver an intensity of 600 W/m² to the samples, and        samples were monitored for changes in optical absorption of the        dye. Optical absorption was followed with a Gretag Spectrolino        reflectance spectrometer. The observations are summarized in        FIG. 8. These results show that the use of an oxygen barrier        more than doubles the amount of time required to achieve 50%        reduction in the peak optical density from ˜500 hours to ˜1100        hours.

Example 6 Impact of Multilayer Stabilized Energy Conversion Element byComparison to Non-Stabilized Structures: Simulated Sunlight Exposure

-   -   Film E: An Energy Conversion Layer was prepared as in Example 1        except that 3,4,9,10-perylenetetracarboxylic acid        bis(2,6-diisopropyl)anilide was used as the dye.    -   Laminate 6: Film E was heat laminated at 266° F. to a 10 mil        clear polyester carrier and the release base was removed to        yield an energy conversion element without a stability        enhancement layer or a protective layer.    -   Laminate 7: A portion of Film B was heat laminated to a portion        of Laminate 6 at 266° F. to yield an energy conversion element        with a protective layer comprising UV protection.    -   Laminate 8: To a second portion of Laminate 6 was heat laminated        a portion of Film C using a thermal adhesive. A second portion        of Film B was laminated to the top (PVA) surface, to yield an        energy conversion element with a stability enhancement layer and        a protective layer comprising UV absorbers.        Portions of Laminate 6, Laminate 7, and Laminate 8 were        subjected to continuous exposure in a sunlight simulator using a        full spectrum 1800 W xenon lamp filtered with a Daylight-Q        optical filter to deliver an intensity of 600 W/m² to the        samples, and samples were monitored for changes in optical        absorption of the dye. Optical absorption was followed with a        UV/Vis spectrophotometer, and digitized spectra were corrected        for optical scatter of the samples. The observations are        summarized in FIG. 9. These results show that the use of a UV        absorber in a protective layer doubles the time required to        achieve 20% reduction in the peak optical density from ˜100        hours to ˜200 hours. However, with the use of the stability        enhancement layer in addition to the protective layer, an 8-fold        stability enhancement is observed since the same 20% reduction        in dye density is seen only after ˜1600 hours of continuous        exposure.

Example 7 Conversion of LED Emission Using Multiple Energy ConversionLayers, Stability Enhancement Layer and Protective Layer

-   -   Film F: Energy Conversion Layer #1: A formulation containing        7.95 parts acrylic copolymer, such as Neocryl® B-805, 0.0200        parts diisobutyl 4,10-dicyanoperylene-3,9-dicarboxylate, 0.103        parts defoamer, such as Foamex N, 0.0207 parts wetting agent,        such as Noresil S-900, 7.95 parts toluene, and 3.99 parts        dioxolane were prepared and mixed for 30 minutes at room        temperature. The solution was coated on release base at 10 mil        or 250 micron wet coating thickness, dried at 35° C. for 12 hour        and then at 80° C. for an additional 12 hours, to yield a 3 mil        or 75 microns thick, yellow-green film.    -   Film G: Energy Conversion Layer #2: A formulation containing        6.958 parts of acrylic copolymer, such as Elvacite 2014, 0.00458        parts        1,6,7,12-tetraphenoxy-N,N-di(2,6-diisopropylphenyl)-3,4:9,10-perylenediimide,        0.0200 parts wetting agent, such as Noresil S-900, 0.100 parts        defoamer, such as Foamex N, 10.835 parts toluene, and 4.589        parts dioxolane were prepared and mixed for 30 minutes at room        temperature. The solution was coated on release base at 20 mil        or 500 microns wet thickness, dried at 35° C. for 12 hours and        then at 80° C. for an additional 12 hours, to yield a 7 mil or        175 microns thick, pink film.    -   Laminate 9: Film F, Energy Conversion Layer #1, was heat        laminated at 266° F. to a 10 mil clear polyester carrier and the        release base removed.    -   Laminate 10: Film G, Energy Conversion Layer #2, was laminated        to Laminate 9 at 266° F. and its release base removed.    -   Laminate 11: Film C, Stability Enhancement layer, was laminated        at 266° F. to the top of Laminate 10 using a thermal adhesive.    -   Laminate 12: Film B, Protective Layer with photostabilizers, was        laminated at 266° F. to the top of Laminate 11 using a thermal        adhesive.        Laminate 12 was illuminated with the emission from a deep blue        LED operating at 450 nm to give a cool white light. The spectrum        of the emission is shown in FIG. 10.

Example 8 Preparation of Multilayer Stabilized Energy Conversion Layerwith Selective Reflection Layer for Use with an LED Primary EmissionSource

-   -   Selective reflection films with desired transmissive and        reflective properties can be sourced from a number of        manufacturers of interference films on a variety of substrates,        including polyester sheets. These filters can be constructed        entirely of dielectric materials, leading to filters that        reflect a significant amount of visible light with low        absorption loss. An interference film designed to transmit light        at wavelengths less than 470 nm and reflect light greater than        470 nm was obtained to serve as a base material onto which        energy conversion layers, such as those described in Example 1,        can be coated or laminated, with further stability enhancement        layer and protective layers, such as described in Examples 2 and        3, also applied. Such a structure can be pumped with a deep blue        LED with an emission wavelength of about 450 nm, to produce a        white light emission from the combination of the source LED and        converted emissions. By way of example, energy conversion layers        were prepared using 3-cyanoperylene-9,10-dicarboxylic acid        2′,6′-diiosopropylanilide at several different concentrations in        Neocryl® B805 resin. A diffusion layer prepared from 18 micron        diameter hollow glass spheres dispersed in Neocryl® B805 was        laminated to each sample. Each sample was exposed to a deep blue        LED source both without and with a selective reflection layer        between the source and the sample. The results are shown in        Table 3. The results clearly demonstrate the improvement in the        amount of forward propagating light collected, with greater        effects observed as amount of converted light in the total        spectrum increases.

TABLE 3 Emission w/o Emission w/ Improvement Dye reflection layerreflection layer due to reflection Concentration (mW/cm²) (mW/cm²) layer0.10% 21.62 23.19  7% 0.30% 13.73 16.66 21% 0.50% 9.68 12.47 29%

Example 9 Preparation of Multilayer Stabilized Energy Conversion ofPersistent Phosphor Emission: Integrated Phosphor and Energy ConversionLayers

-   -   The energy conversion of emission from high persistent phosphors        was described with examples in U.S. Patent Application Ser. No.        2008/0185,557, which is incorporated by reference herein in its        entirety and which teaches that fluorescent dyes can be combined        with dispersed phosphor in a single layer. The utility of the        energy storage and conversion element can be enhanced by        including an additional stability enhancement layer and a        protective layer that includes a UV absorber. The energy storage        and conversion layer can furthermore be prepared on a white or        aluminized base substrate to provide additional broadband        reflection to maximize front surface emission.

Example 10 Preparation of Multilayer Stabilized Energy Conversion ofPersistent Phosphor Emission: Laminated Phosphor and Energy ConversionLayers

-   -   The energy storage and conversion layer described in Example 9        can be constructed in a laminate form by separating the phosphor        layer (energy storage) from the energy conversion layer.        Furthermore, the energy conversion layer itself can be        deconstructed into a series of separate layers, as described in        Example 1, each of which perform a step of the desired energy        cascade. The energy storage and conversion layers can then be        further stabilized as described in Example 9 by the addition of        stability enhancement layer and protective layers. The emission        from the front surface can be maximized by the use of a        reflective substrate, as described in Example 9.

Example 11 Preparation of Multilayer Stabilized Energy Conversion ofChemiluminescent Emission

-   -   An energy conversion layer, such as described, may be formed        into a cylindrical sleeve, such as illustrated in FIG. 6, so as        to contain within it a chemiluminescent device, such as a glow        stick. The inner and outer surfaces can be coated with materials        so as to separately form a stability enhancement layer and a        protective layer, to give a multilayer, stabilized cylindrical        energy conversion element.

In another embodiment of the present teachings, the multilayer structurecan be used as a device for authentication. In one instance, themultilayer structure comprises at least one energy conversion layer andat least one stability enhancement layer. The energy conversion layercomprises one or more photoluminescent materials that converts a primaryelectromagnetic radiation to a longer output wavelength. The stabilityenhancement layer increases the photolytic and thermal stability of theenergy conversion layer. The multilayer structure may further comprise areflection layer which redirects radiation emitted in the energyconversion layer, for example in the direction where in may beperceived. The multilayer structure may further comprise a blockinglayer that prevents at least a portion of the energy conversion layerfrom converting the primary electromagnetic radiation to a longer outputwavelength. The blocking layer can be rendered as a discrete layer orwithin the energy conversion layer itself. The blocking layer can be inthe form of a transparent polymer film comprising dyes such as, but notlimited to Tinuvin® 479, Tinuvin® 327, Tinuvin® 405, and Uvitex OB. Themultilayer structure may further comprise an optical variable elementthat controls the output wavelength emission as a function of viewingangle. The optical variable element can be applied to the multilayerstructure in a number of ways, such as, but not limited to, a discreteseparate layer or through embossing the surface of at least one layer ofthe multilayer structure. Examples of optical variable elements include,but not limited to, lenticular films and microprismatic films. Themultilayer structure may further comprise a protective layer thatenhances durability of at least the energy conversion layer.

Regarding the multilayer structure herein and above, the use of themultilayer structure for authentication can include applying themultilayer structure onto or into at least a portion of an object,wherein the multilayer structure is exposed to a primary electromagneticenergy and an output wavelength emission is detected. Detection of theoutput wavelength emission can be performed in various ways, such as,but not limited to, visual detection or spectroscopic detection. The useof the multilayer structure may further include comparing the outputwavelength emission to a known output wavelength emission associatedwith the multilayer structure. The multilayer structure can belaminated, thermally or with adhesive, onto any item requiringauthentication. In another instance, the energy conversion layer can beapplied image-wise using a variety of printing methods including screenprinting and ink jet printing. In yet another instance, the blockinglayer is applied image-wise so there are areas that block the chargingenergy and areas that allow the charging energy to charge the phosphorlayer. The blocking layer can be applied image-wise using a variety ofprinting methods including screen printing and ink jet printing.

For the purposes of describing and defining the present teachings, it isnoted that the term “substantially” is utilized herein to represent theinherent degree of uncertainty that may be attributed to anyquantitative comparison, value, measurement, or other representation.The term “substantially” is also utilized herein to represent the degreeby which a quantitative representation may vary from a stated referencewithout resulting in a change in the basic function of the subjectmatter at issue.

Although the teachings have been described with respect to variousembodiments, it should be realized these teachings are also capable of awide variety of further and other embodiments within the spirit andscope of the appended disclosure.

1. A multilayer structure for authentication, said multilayer structurecomprising: an energy conversion layer that converts energy of a primaryelectromagnetic radiation to a longer output wavelength, said energyconversion layer comprising: a polymer and a first photoluminescentmaterial with a first radiation absorption spectrum that at leastpartially overlaps with the primary electromagnetic radiation; at leastone stability enhancement layer that increases the photolytic andthermal stability of said multilayer structure; and at least one opticalvariable element that varies the longer output wavelength that isobserved as a function of viewing angle.
 2. The multilayer structure ofclaim 1, wherein said multilayer structure further comprises a secondphotoluminescent material with a second radiation absorption spectrumwhich at least partially overlaps with the emission spectrum of thefirst photoluminescent material.
 3. The multilayer structure of claim 1,further comprising a reflection layer, said reflection layer redirectsradiation emitted in said energy conversion layer to the viewinghemisphere.
 4. The multilayer structure of claim 3, wherein saidreflection layer transmits at least a portion of the primaryelectromagnetic radiation.
 5. The multilayer structure of claim 3,wherein said reflection layer additionally fulfills the function of saidat least one stability enhancement layer.
 6. The multilayer structure ofclaim 3, wherein said reflection layer is rendered as either a discreteparticle or a discrete element.
 7. The multilayer structure of 3,wherein said reflection layer comprises a plurality of alternate layersof non-metallic materials with high and low dielectric constants.
 8. Themultilayer structure of 3, wherein said reflection layer is a lenticularelement or a microprismatic element.
 9. The multilayer structure ofclaim 1, further comprising a diffusion layer, said diffusion layerincreases optical scattering.
 10. The multilayer structure of claim 9,further comprising a reflection layer, wherein said reflection layerredirects radiation emitted in at least said energy conversion layer.11. The multilayer structure of claim 9, wherein said diffusion layer isrendered as a discrete particle.
 12. The multilayer structure of claim1, further comprising at least one blocking layer, said at least oneblocking layer prevents at least a portion of said energy conversionlayer from converting energy of the primary electromagnetic radiation tothe longer output wavelength.
 13. The multilayer structure of claim 12,wherein said at least one blocking layer is rendered as a discreteparticle.
 14. The multilayer structure of claim 12, wherein said atleast one blocking layer is at least one selectively reflective materialor at least one selectively absorbing material.
 15. The multilayerstructure of claim 12, wherein said at least one blocking layer isrendered within said energy conversion layer.
 16. The multilayerstructure of claim 1, further comprising a protective layer, saidprotective layer provides mechanical and chemical durability for saidmultilayer structure.
 17. The multilayer structure of claim 1, whereinthe polymer and the first photoluminescent material in said energyconversion layer comprise a solid state solution.
 18. The multilayerstructure of claim 1, wherein the primary electromagnetic radiation isinfrared, visible, ultraviolet, or any combination thereof.
 19. Themultilayer structure of claim 1, wherein the longer output wavelength isinfrared, visible, ultraviolet, or any combination thereof.
 20. Themultilayer structure of claim 1, wherein the primary electromagneticradiation emanates from within said multilayer structure.
 21. Themultilayer structure of claim 1, wherein the primary electromagneticradiation is ambient light.
 22. The multi layer structure of claim 1,wherein said energy conversion layer further comprises a highpersistence photoluminescent material, and wherein the primaryelectromagnetic radiation emanates from said high persistencephotoluminescent material.
 23. The multi layer structure of claim 1,wherein the primary electromagnetic radiation is emanated from anelectroluminescent source, a solid state device, a chemiluminescentsource, or any combination thereof.
 24. The multilayer structure ofclaim 1, wherein the primary electromagnetic radiation emanates from awaveguide that is either coupled or decoupled to an electroluminescentsource.
 25. The multilayer structure of claim 1, wherein said energyconversion layer further comprises a light scattering element.
 26. Themultilayer structure of claim 1, wherein said energy conversion layer isrendered as a discrete particle.
 27. The multilayer structure of claim1, wherein one of said at least one stability enhancement layer isrendered on a first side of said energy conversion layer and another oneof said at least one stability enhancement layer is rendered on anotherside of said energy conversion layer.
 28. The multilayer structure ofclaim 1, wherein said at least one stability enhancement layer isrendered as a discrete particle.
 29. The multilayer structure of claim1, wherein said energy conversion layer further comprises a singletoxygen quencher.
 30. The multilayer structure of claim 1, wherein saidat least one optical variable element is rendered as a discreteparticle.
 31. The multilayer structure of claim 1, wherein said at leastone optical variable element comprises a plurality of alternate layersof non-metallic materials with high and low dielectric constants. 32.The multilayer structure of claim 1, wherein said at least one opticalvariable element is rendered within at least one layer of saidmultilayer structure.
 33. A method of fabricating a multilayer structurefor authentication, said method comprising: forming an energy conversionlayer comprising a polymer and a first photoluminescent material with afirst absorption spectrum that at least partially overlaps with aprimary electromagnetic radiation; overlaying at least one stabilityenhancement layer over at least one side of said energy conversion layerthat increases the photolytic and thermal stability of said multilayerstructure; and applying at least one optical variable element into oronto at least a portion of at least one layer of said multilayerstructure that varies a longer output wavelength that is observed as afunction of viewing angle.
 34. The method of claim 33, wherein saidmultilayer structure further comprises a second photoluminescentmaterial with a second radiation absorption spectrum that at leastpartially overlaps with the emission spectrum of the firstphotoluminescent material.
 35. The method of claim 33, furthercomprising applying a diffusion layer disposed over at least one side ofsaid energy conversion layer, wherein said diffusion layer increasesoptical scattering.
 36. The method of claim 35, further comprisingrendering a reflection layer disposed over one side of said diffusionlayer, wherein said reflection layer redirects radiation emitted in atleast said energy conversion layer.
 37. The method of claim 33, furthercomprising rendering a reflection layer disposed over one side of theenergy conversion layer, wherein said reflection layer redirectsradiation emitted in said energy conversion layer.
 38. The method ofclaim 33, further comprising rendering at least one blocking layerdisposed into or over at least a portion of said energy conversionlayer, wherein said at least one blocking layer prevents at least aportion of said energy conversion layer from converting the primaryelectromagnetic radiation to the longer output wavelength.
 39. Themethod of claim 33, further comprising overlaying a protective layerover a surface of said at least one stability enhancement layer, thesurface not having another layer disposed on the surface, wherein saidprotective layer protects said multilayer structure physically andchemically.
 40. A method of identifying an object for authentication,said method comprising: applying a multilayer structure onto or into atleast a portion of the object, the multilayer structure comprising: (i)an energy conversion layer that converts the energy of a primaryelectromagnetic radiation to a longer output wavelength, and (ii) atleast one stability enhancement layer that increases the photolytic andthermal stability of said multilayer structure, and (iii) at least oneoptical variable element that varies the longer output wavelength thatis observed as a function of viewing angle, and wherein said multilayerstructure is exposed to the primary electromagnetic radiation and thelonger output wavelength is detected.
 41. The method of claim 40,wherein applying is incorporating said multilayer structure into theobject during manufacture, building said multilayer structure on theobject, later affixing said multilayer structure to the object, or anycombination thereof.
 42. The method of claim 40, wherein said multilayerstructure further comprises a reflection layer, said reflection layerredirects radiation emitted in said energy conversion layer.
 43. Themethod of claim 40, wherein said multilayer structure further comprisesa diffusion layer, said diffusion layer increases optical scattering.44. The method of claim 40, wherein said multilayer structure furthercomprises at least one blocking layer, said at least one blocking layerprevents at least a portion of said energy conversion layer fromconverting energy of the primary electromagnetic radiation to the longeroutput wavelength.
 45. The method of claim 40, wherein said multilayerstructure further comprises a protective layer, said protective layerprovides physical and chemical durability for said multilayer structure.46. The method of claim 40, wherein the primary electromagneticradiation is infrared, visible, ultraviolet, or any combination thereof.47. The method of claim 40, wherein the longer output wavelength isinfrared, visible, ultraviolet, or any combination thereof.
 48. Themethod of claim 40, wherein detected is observing the longer outputwavelength visually, observing the longer output wavelengthspectroscopically, or any combination thereof.